Touch Screens

ABSTRACT

A touch screen control system comprising a touch screen ( 14 ) having first ( 50 ) and second conductive layers ( 52 ) arranged to be brought together by touching of the screen ( 14 ). A detection system ( 20, 30 ) is arranged to detect a contact position at which the screen ( 14 ) is touched by monitoring electrical signals from at least one of the layers ( 50, 52 ). The system further comprises an antenna ( 26 ), and the detection system includes a proximity sensing signal generator arranged to generate a proximity sensing signal to be transmitted between the antenna ( 26 ) and the first layer ( 50 ) via a user ( 40 ) of the system. The detection system is further arranged to receive the transmitted proximity sensing signal and determine therefrom the distance between a part ( 46 ) of the user and the touch screen ( 14 ).

This invention relates to touch screens, and has particular applicationin touch screens arranged for use in the control of vehicle sub-systems.

It is known to provide touch screens in vehicles to allow the driver orpassenger to control various sub-systems of the vehicle. It is alsoknown to provide proximity sensing systems in which a signal istransmitted through the driver or passenger, for example between anantenna in their seat and a control input such as a push button or touchscreen, and the strength of the received signal measured. The strengthof the signal indicates the capacitance between the driver or passengerand the control input, and therefore the distance of the user's handfrom the control input. Typically the signal is received at, ortransmitted from the control input by means of a specially providedantenna, such as a conductive coating on the push button, or aconductive bezel around the touch screen.

According to the invention, there is provided a touch screen controlsystem comprising a touch screen having first and second conductivelayers arranged to be brought together by touching of the screen, and adetection system arranged to detect a contact position at which thescreen is touched by monitoring electrical signals from at least one ofthe layers, wherein the system further comprises an antenna, and thedetection system includes a proximity sensing signal generator arrangedto generate a proximity sensing signal to be transmitted between theantenna and the first layer via a user of the system, and the detectionsystem is further arranged to receive the transmitted proximity sensingsignal and determine therefrom the distance between a part of the userand the touch screen.

The first layer may have two contact elements extending along oppositesides thereof, and the detection system may be arranged to connect thecontact elements to different potentials so that the potential of thelayer varies with the distance from each of the two contact elementsthereby to enable sensing of the contact position. Such an arrangementis consistent with a convenient four-wire touch screen design.

The detection system may be arranged to transmit the proximity sensingsignal from the first layer to the antenna, or from the antenna to thefirst layer. The detection system may be arranged to transmit, orreceive, the proximity sensing signal via at least one of the contactelements, and preferably both of the contact elements.

Where the proximity sensing signal is transmitted to the first layer,the detection system preferably includes a summing device arranged tosum signals from the two contact elements to produce a receivedproximity sensing signal.

The detection system may be arranged to control at least one connectionto the second layer such that it acts as a shield for at least part ofthe time when the proximity sensing signal is being received. Forexample, the detection system may be arranged to determine when thetouch screen is being touched, and to control said at least oneconnection to connect the second layer to at least one fixed potentialwhen the proximity sensing signal is being received and the touch screenis not being touched.

Conveniently, said at least one connection may comprise the normalconnections to the second layer used for touch position sensing. Forexample they may comprise two connections arranged to connect the secondlayer between two different potentials, both when the proximity sensingsignal is being received and the touch screen is not being touched, andwhen the detection system is determining the contact position.

The detection system may be arranged to electrically isolate the secondlayer when the proximity sensing signal is being received and the touchscreen is being touched. This avoids potentials on the second layerinterfering with the proximity sensing signal received on the firstlayer.

The detection system may be arranged to alternate between a touchposition sensing mode, in which it is arranged to determine the contactposition, and a proximity sensing mode, in which it is arranged tomeasure said distance.

The detection system may comprise a touch screen controller arranged todetect the contact position, and a proximity sensing system includingthe proximity sensing signal generator. The touch screen controller maythen be arranged to receive said electrical signals, including theproximity sensing signal from the touch screen and the proximity sensingsystem may be arranged to receive the proximity sensing signal from thetouch screen controller. The touch screen controller may also bearranged to send a synchronisation signal to the proximity sensingsystem to enable the proximity sensing system to determine when it isreceiving the proximity sensing signal from the touch screen controller.

The present invention further provides a touch screen control systemcomprising a touch screen having first and second conductive layersarranged to be brought together by touching of the screen, and adetection system arranged to detect a contact position at which thescreen is touched by monitoring electrical signals from at least one ofthe layers, wherein the first layer is arranged to act as a receivingantenna to receive a proximity sensing signal transmitted from atransmitting antenna via a user, and the detection system is furtherarranged to transmit the received proximity sensing signal on to aproximity sensing system thereby to enable the proximity sensing systemto determine a distance between a part of the user and the touch screen.

Preferred embodiments of the present invention will now be described byway of example only and with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic system diagram of a control system according to afirst embodiment of the invention;

FIG. 2 is a schematic side view of the system of FIG. 1 incorporated ina vehicle;

FIG. 3 is a schematic circuit diagram of the system of FIG. 1;

FIG. 4 is a schematic circuit diagram of the system of FIG. 1 arrangedto detect touching of its touch screen;

FIG. 5 is a schematic circuit diagram of the system of FIG. 1 arrangedto determine an x coordinate of a point of contact on the touch screen;

FIG. 6 is a schematic circuit diagram of the system of FIG. 1 arrangedto determine a y coordinate of a point of contact on the touch screen;

FIG. 7 is a schematic circuit diagram of the system of FIG. 1 arrangedfor proximity sensing when the touch screen is not being touched;

FIG. 8 is a schematic circuit diagram of the system of FIG. 1 arrangedfor proximity sensing when the touch screen is being touched;

FIG. 9 is a schematic circuit diagram of the system of FIG. 1 showingEMC and ESD protection;

FIG. 10 is a diagram showing communication between two control units ofthe system of FIG. 1;

FIG. 11 is a timing diagram showing the signals at various points in thesystem of FIG. 1 during operation;

FIG. 12 is circuit diagram showing details of a touch screen switchingand protection circuit of the system of FIG. 1;

FIG. 13 is circuit diagram showing details of a touch screen inputbuffer circuit of the system of FIG. 1; and

FIG. 14 is circuit diagram showing a circuit for generating a referencesignal in the system of FIG. 1.

Referring to FIGS. 1 and 2, a control system for a vehicle subsystem 10,which in this particular embodiment is a heating and ventilating system,comprises a system controller 12 arranged to control operation of thesubsystem 10, and a four-wire touch screen control panel 14, which hasan LCD display 15 behind it. A touch screen controller 20 controlsoperation of the touch screen 14 and detects when the touch screen hasbeen touched, and in what position, as will be described below. Thetouch screen 14 is used to control the operation of the subsystem 10,and the system controller 12 receives input signals from the touchscreen controller 20.

A proximity sensing system for detecting when a user's hand isapproaching the touch screen 14 includes a driver's seat antenna 26 inthe form of a conductive sheet provided in the driver's seat 29. Theantenna 26 is connected to a proximity sensing processor or ECU 30,which is arranged to transmit electrical signals through the seatantenna 26 and receive them via the touch screen 14 and the touch screencontroller 20, as will be described in detail below. The strength ofthese signals is used to determine the distance between a part of thedriver's body, usually his hand, and the touch screen 14. A similar seatantenna 28 is provided in the front passenger seat of the vehicle.

The proximity sensing ECU 30 is also connected to a graphics controller34, which in turn is connected to a head-up display unit 36 mountedbeneath the vehicle windscreen 38 and arranged to project a display ontothe windscreen so that it can be viewed by the driver 40 from thedriver's seat 29. The proximity sensing ECU 30 is also connected to abus interface 33 so that it can use data available on the vehicle CANbus.

The general operation of the proximity sensing system will now bedescribed. The proximity sensing ECU 30 applies a sinusoidal signal at apredetermined frequency and amplitude, in this case 67 kHz and 10V, tothe driver's seat antenna 26, and monitors the signals received from thetouch screen via the touch screen controller 20 at that frequency. Whilethe driver 40 is not operating the touch screen 14 provided no part ofhis body is within a predetermined region 44 close to the touch screen14, then no signal at 67 kHz will be detected from the touch screen.Under these conditions the head-up display unit 36 is not operated andthe controlled system 10 is not altered.

If the driver 40 brings his hand 46 into the region 44 then the signalfrom the seat antenna 26 will be transmitted through his body to thetouch screen where it will be received sufficiently strongly to bedetected by the proximity sensing ECU 30. This is because the body ofthe driver 40 has different dielectric properties from the surroundingair and is better able to transmit the signal from the driver's seatantenna 26 to the touch screen. When the signal is received from thedriver's seat antenna 26, the proximity sensing ECU 30 activates thehead-up display unit 36 which projects onto the windscreen a HUD image37 representing the current status of the controlled system. When thedriver's hand 46 touches the touch screen 14, this is detected by thetouch screen controller 20 and notified to the proximity sensing ECU 30which modifies the image 37 to indicate to the driver 40 which of thecontrol areas 16 his hand is touching, in this case by highlighting therepresentation of that control area. The head-up display unit 36 altersthe HUD image 37 to indicate to the driver the effect of his inputs tothe controlled system. When the driver 40 has altered the controlledsystem to the desired setting, he withdraws his hand 46. When his handleaves the region 44, the touch screen 14 ceases to receive the signalfrom the driver's seat antenna 26, and the proximity sensing ECU 30detects the removal of the driver's hand. It then de-activates thehead-up display unit 36.

In order to distinguish between the driver and the passenger, theproximity sensing system is arranged to transmit the proximity sensingsignal alternately through the driver's seat antenna 26 and thepassenger's seat antenna 28.

Referring to FIG. 3, the touch screen 14 comprises a front layer 50,which is on the outside of the screen 14 nearest to the driver andpassenger, and a rear layer 52 between the front layer 50 and the LCDdisplay 15. The front and rear layers 50, 52 are electricallyconductive, parallel to each other and spaced apart so that there is agap between them. Two contact wires 54, 56 extend along the left andright edges 58, 60 of the front layer 50, and two further contact wires62, 64 extend along the top and bottom edges 66, 68 of the rear layer52. Each of the four contact wires 54, 56, 62, 64 is connectedindividually to an analogue to digital converter (ADC) in the touchscreen controller 20, which uses the signals on the contact wires todetermine the position of the contact point at which the touch screen istouched. The top and bottom contact wires 62, 64 of the rear layer 52are also connected to a 5V supply and to ground respectively by switchesS1 and S2, and the right and left contact wires 56, 54 of the frontlayer 50 are also connected to the 5V supply and ground respectively byswitches S3 and S4. The left and right contact wires 54, 56 of the frontlayer 50 are also both connected to a summing circuit 70, the output ofwhich is input to a buffer 72 which has a 5V supply. The output 73 fromthe buffer 72 is used as an input to the proximity sensing controllerwhich uses it for proximity sensing.

The bottom contact wire 64 on the rear layer 52 is also connected toground through a resistor 74 and a further switch S5. The output 73 fromthe buffer 72 is connected via a switch S6 to a battery voltage V_(BATT)which is greater than 8V. This is used to produce a timing signal toco-ordinate operation of the touch screen controller 20 and theproximity sensing controller 30.

Referring to FIG. 4, in order to determine whether or not the touchscreen 14 is being touched, the touch screen switching circuit is putinto a touch detection state. In this state the right contact wire 56 ofthe front layer 50 is connected to the 5V supply by the switch S3 andthe left contact wire 54 is disconnected from ground. The whole of thefront layer 50 is therefore at approximately 5V. The lower contact wire64 of the rear layer 52 is connected to ground via the resistor 74 andswitch S5, and the upper contact wire 62 is disconnected from the 5Vsupply. If the screen is not being touched, the voltage of the rearlayer 52 remains at zero. However, if the screen is touched so the frontlayer 50 is moved into contact with the rear layer 52 at any point, thevoltage of the rear layer 52 will rise above zero, which can be detectedby the touch screen controller 20.

Referring to FIG. 5, in order to determine the x-coordinate of theposition in which the touch screen is being touched, the touch screenswitching circuit is put into an x-coordinate sensing state. In thisstate the right contact wire 56 of the front layer 50 is connected tothe 5V supply by the switch S3 and the left contact wire 54 of the frontlayer 50 is connected to ground by the switch S4. This causes the wholeof the right hand side 60 of the front screen to be at 5V, the whole ofthe left hand side 58 to be at substantially zero volts, and allpositions on the front layer 50 to be at a potential which depends ontheir distance, in the horizontal direction, from each of the two sidecontact wires 54, 56. The voltage at the top and bottom contact wires62, 64 is input to the high impedance input of the touch screencontroller 20. If the screen is being touched so that the two layers 50,52 are in contact, then the whole of the rear screen will be brought tothe potential of the point of contact on the front screen. Thispotential is detected by the touch screen controller and indicates thex-coordinate of the point of contact.

Referring to FIG. 6, the process for determining the y coordinatecorresponds to that for determining the x coordinate. The touch screenswitching circuit is put into a y-coordinate sensing state, in which therear layer 52 is connected to the 5V supply at the top contact wire 62and to ground at the bottom contact wire 64 so that the potential on therear layer varies with vertical position. The voltage on the frontscreen is then monitored using the left and right contact wires 54, 56to determine the y coordinate of the point of contact.

Referring to FIG. 7, in order to perform the proximity sensing, thefront layer 50 of the touch screen 14 is used as a receiver antenna forthe proximity sensing signal which is transmitted from the driver orpassenger seat antenna 26, 28 through the driver or passenger. Asindicated above, the strength of the received signal is indicative ofthe distance between the hand of the driver or passenger and the touchscreen 14. To detect the proximity sensing signal, the output from thebuffer 72, is monitored to determine magnitude of its 67 kHz component,which is a measure of the strength of the received proximity sensingsignal at the front layer 50.

When the touch screen 14 is not being touched, for the system to performthe proximity sensing function the touch screen switching circuit is putinto a first proximity sensing state. In this state the rear layer 52 isused as a shield, to shield the front layer 50 from interference fromthe LCD 15. In order to perform this function most effectively the rearlayer 52 needs to be connected to a low impedance source. The topcontact wire 62 is therefore connected to the 5V supply via the switchS1, and the bottom contact wire 64 is connected to ground via the switchS2. For effective shielding, the output impedance of the 5V source andthe impedances of the switches S1 and S2 must both be low at 67 kHz.Ideally the effective impedance at 67 kHz between the rear layer 52 andground should be no more than 100Ω. The effective impedance can becalculated by adding the output impedance of the 5V source to theimpedance of S1 and S2 in parallel. The input impedance of the proximitysensing circuit in the proximity sensing ECU 30 is also arranged to below, in this case approximately 100Ω at 67 kHz, to minimize the effectof the capacitance between the layers of the touch screen 14.

Referring to FIG. 8, the proximity sensing system also needs to functionwhen the screen is being touched. This is so that it can determine whichuser is touching the screen. This can be achieved, for example, bytransmitting different proximity sensing signals through the two seatantennae 26, 28, or by transmitting the signals at different timesthrough the two antennae, in which case the same signal can be used forboth users. When the screen 14 is being touched, the rear layer 52cannot be connected to the 5V supply and ground, as the voltage on therear layer 52 would then interfere with the received proximity sensingsignal on the front layer 50. Therefore in a second proximity sensingstate the rear layer is electrically isolated, being disconnected fromthe 5V supply and from ground by opening the switches S1 and S2. Theproximity sensing signal is received on the front layer 50 and measuredin the same way as when the screen is not being touched. The isolationof the rear layer 52 prevents it from acting as an effective shield, butbecause the screen is being touched, the proximity sensing signal istransmitted strongly through the user to the front layer 50, and cantherefore easily be detected.

Referring to FIG. 9, a capacitor C8, C9 is connected between ground andeach of the left and right contact wires 54, 56 on the front layer 50.Similarly a capacitor C6, C7 is connected between ground and each of thetop and bottom contact wires. These capacitors C6, C7, C8, C9 arefiltering capacitors for electromagnetic compatibility (EMC), and areused to improve the immunity of the touch screen to interference, inparticular from the display 15. The values of these capacitors C6, C7,C8, C9 depend on the resistance of the touch screen 14 and the requiredmeasurement time. Typically C8 and C9 will be at least 100 nF, and inthis case they are 100 nF. Typically C6 and C7 will be a similar valueor lower, and in this case they are also 100 nF. However, the capacitorsC8, C9 on the front layer 50 would interfere with detection of theproximity sensing signal, and the capacitors C6, C7 on the rear layer 52would similarly interfere when the screen is being touched, as theywould then provide a path from the front layer 50 to ground. Thereforethe capacitors C8, C9 are connected to ground through a further switchS7, so that they can be switched out during proximity sensing, and thecapacitors C6, C7 are connected to ground though a further switch S8, sothat they can be switched out during proximity sensing when the screen14 is being touched. Further capacitors of C2, C3, C4, C5 are thereforepermanently connected between each of the contact wires 54, 56, 62, 64and ground. These are of a much lower value, in this case 2.2 nF, andprovide a degree of EMC protection during proximity sensing, withoutsignificantly interfering with the proximity sensing signal.

Referring to FIG. 10, the touch screen controller 20 and the proximitysensing ECU 30 are connected to each other by a coaxial cable 80, theinner 82 of which transmits signals from the touch screen controller 20to the proximity sensing controller 30, and the outer 84 of which isconnected to ground. This coaxial cable is used to transmit the detectedproximity sensing signal from the front layer 50 of the touch screen tothe proximity sensing controller 30, and also to transmit asynchronisation signal, so that the proximity sensing controller candetermine when the signal on the coaxial cable is the proximity sensingsignal, and when it is not. It is also used by the touch screencontroller 20 to indicate to the proximity sensing ECU 30 when the touchscreen 14 is being touched. The touch screen controller 20 alternatesbetween a touch screen mode, in which it detects the presence or absenceof a touch on the touch screen, and the position of any touch, and aproximity sensing mode, in which it receives the proximity sensingsignal and transmits it on to the proximity sensing controller 30. Atpower up the proximity sensing controller synchronises itself to thesynchronisation signal, and remains in synchronisation thereafter.

Referring to FIG. 1, the touch screen controller 20 in this exampleoperates on the basis of a cycle time of 32 ms, made up of a touchscreen operation cycle of 12 ms and a proximity detection cycle of 20ms. These cycle times could obviously vary. During this cycle each ofthe switches S1 to S6 can be opened and closed by applying a controlsignal to them, as shown in FIG. 11, the control signal being high toclose the switch and low to open it. The proximity sensing signal on thecoaxial cable is shown at the bottom of FIG. 11.

At the beginning of the touch screen cycle, switches S3 and S5 areclosed for a predetermined time, putting the switching circuit into thetouch detection state, and the signals from the two rear layer contactwires 62, 64 are monitored by the proximity sensing controller 20 todetermine whether the screen is being touched. At the end of this timeswitch S6 is closed to produce a rising edge in the signal on thecoaxial cable 80. This is used by the proximity sensing controller forsynchronisation. The switch S6 is then opened again after either afirst, short period, if touching of the touch screen has been detected,or a second longer period, of touching of the touch screen has not beendetected. The length of the V_(BATT) synchronisation pulse thereforeindicates to the proximity sensing controller 30 whether the screen 14is being touched or not. This enables the proximity sensing controllerto adjust its sensitivity to take account of the fact that the proximitysensing signal will be received much more strongly when the screen 14 isbeing touched. It also enables the proximity sensing controller 30 tomodify its inputs to the system controller 12, if appropriate, when thescreen 14 is being touched.

After the end of the synchronisation pulse, the remainder of the 12 mstouch screen cycle is taken up by the touch screen controller 20determining the position of the point at which the touch screen is beingtouched. This is carried out in a conventional manner by putting thetouch screen switching circuit into the x-coordinate and y-coordinatesensing states described above. During this part of the cycle the signalon the coaxial cable 80 will vary in a number of ways, and the proximitysensing controller is arranged to ignore it. The signal will never riseabove 5V, apart from during the synchronisation pulse, because the powersupply to the buffer 72 is at 5V, i.e. lower than the voltage of thesynchronisation signal. This ensures that the synchronisation signal canbe distinguished from all other signals on the coaxial cable 80.

Then at the beginning of the proximity sensing cycle, switches S3 and S4are opened so that the potential on the front layer 50 can vary with theproximity sensing signal transmitted through the user, and switches S5and S6 are kept open. Switches S1 and S2 are closed if the screen 14 isnot being touched, as in the second cycle shown, so that the rear layer52 can act as a shield as described above. This is the first proximitysensing state described above. If the screen 14 is being touched,switches S1 and S2 are opened during the proximity sensing cycle, as inthe first cycle shown. As described above, this prevents the voltage onthe rear layer 52 being altered in a way what would interfere with theproximity sensing signal as detected on the front layer 50. During theproximity sensing cycle, the proximity sensing controller 30 transmitsthe proximity sensing signal through each of the driver and passengerseat antennae 26, 28, alternately and monitors the received signal viathe coaxial cable 80 to determine the proximity of the driver orpassenger's hand to the screen 14, and to determine whether it is thedriver's or the passenger's hand which is being detected.

For effective operation, the input buffer 72 needs to have a high DCinput impedance, preferably at least 10 kΩ. This is to ensure that thetouch detection can work correctly. It also needs to be able to amplifythe proximity sensing signal, which in this embodiment is around 100 nApeak to peak, so that it can be detected by the proximity sensingcontroller 30. The buffer 72 also needs to have a low input impedance atthe frequency of the proximity sensing signal, 67 kHz, preferably lowerthan the reactance between the touch screen layers 50, 52, to preventthe sensitivity of the sensing system from being reduced by thecapacitance of the touch screen 14. For example, for a 10 nF touchscreen, the input impedance needs to be less than 230Ω at 67 kHz.

An example of a suitable switching and protection circuit for the touchscreen 14 is shown in FIG. 12. The two front touch screen layer contactwires 54, 56 are connected to two inputs X1, X2 of a header J2, and thetwo rear layer contact wires 62, 64 to two further inputs Y1, Y2.Capacitors C2 to C5 provide EMC suppression. Capacitors C6 to C9 areswitched into the circuit during touch screen operation, to providefurther EMC suppression during proximity sensing, by transistors Q6 andQ7 which implement switches S7 and S8. Transistors Q1, Q2, Q3, Q4, andQ5 implement switches S1, S3, S5, S2 and S4 respectively. Diodes D1 toD4 provide electrostatic discharge (ESD) protection. The outputs on X1and X2, from the front layer left and right contact wires 54, 56, areconnected to the buffer circuit 72, which is implemented as shown inFIG. 13.

Referring to FIG. 13, in an example of a suitable buffer circuit, C13and C17 couple the AC component of the proximity sensing signals fromthe two front layer contact wires to the virtual ground input of op ampU6A. R35 sets the AC input impedance, and R33 and R39 set the DC inputimpedance. R40, R44 and C18 provide a bias voltage to the circuit of 2V.The ratios of R30 to R33 and R30 to R39 respectively define the DC gainto the signals from each of the front layer contact wires 54, 56. R30and C11 define the sensitivity to AC current.

Op amp U6B and its surrounding components form a 67 kHz band passfilter. Transistors Q8 and Q9 implement switch S6, to pull the output ofthe buffer up to V_(BATT), controlled using a 5V signal.

As shown in FIG. 14, the V_(BATT) reference signal is provided from a12V input by zener diodes D5 and D6, resistors R45 and R46 andtransistor Q10.

1. A touch screen control system comprising a touch screen having firstand second conductive layers arranged to be brought together by touchingof the screen, and a detection system arranged to detect a contactposition at which the screen is touched by monitoring electrical signalsfrom at least one of the layers, wherein the system further comprises anantenna, and the detection system includes a proximity sensing signalgenerator arranged to generate a proximity sensing signal to betransmitted between the antenna and the first layer via a user of thesystem, and the detection system is further arranged to receive thetransmitted proximity sensing signal and determine therefrom a distancebetween a part of the user and the touch screen.
 2. A system accordingto claim 1, wherein the first layer has two contact elements extendingalong opposite sides thereof.
 3. A system according to claim 2, whereinthe detection system is arranged to connect the contact elements todifferent potentials so that the potential of each of the layers varieswith a distance from each of the two contact elements thereby to enablesensing of the contact position.
 4. A system according to claim 1,wherein the detection system is arranged to transmit the proximitysensing signal from the antenna to the first layer.
 5. A systemaccording to claim 2, wherein the detection system is arranged toreceive the proximity sensing signal via at least one of the contactelements.
 6. A system according to claim 5, wherein the detection systemis arranged to receive the proximity sensing signal via both of thecontact elements.
 7. A system according to claim 6, wherein thedetection system includes a summing device arranged to sum signals fromthe two contact elements to produce a received proximity sensing signal.8. A system according to claim 4, wherein the detection system isarranged to control at least one connection to the second layer suchthat the second layer acts as a shield for at least part of the timewhen the proximity sensing signal is being received.
 9. A systemaccording to claim 8, wherein the detection system is arranged todetermine when the touch screen is being touched, and to control said atleast one connection to connect the second layer to at least one fixedpotential when the proximity sensing signal is being received and thetouch screen is not being touched.
 10. A system according to claim 9,wherein said at least one connection comprises two connections arrangedto connect the second layer between two different potentials, both whenthe proximity sensing signal is being received and the touch screen isnot being touched, and when the detection system is determining thecontact position.
 11. A system according to claim 9, wherein thedetection system is arranged to electrically isolate the second layerwhen the proximity sensing signal is being received and the touch screenis being touched.
 12. A system according to claim 1, wherein thedetection system is arranged to alternate between a touch positionsensing mode, in which it is arranged to determine the contact position,and a proximity sensing mode, in which it is arranged to measure saiddistance between a part of the user and the touch screen.
 13. A systemaccording to claim 1, wherein the detection system comprises a touchscreen controller arranged to detect the contact position, and aproximity sensing system including the proximity sensing signalgenerator.
 14. A system according to claim 13, wherein the touch screencontroller is arranged to receive said electrical signals, including theproximity sensing signal from the touch screen, and the proximitysensing system is arranged to receive the proximity sensing signal fromthe touch screen controller.
 15. A system according to claim 14, whereinthe touch screen controller is arranged to send a synchronisation signalto the proximity sensing system to enable the proximity sensing systemto determine when it is receiving the proximity sensing signal from thetouch screen controller.
 16. A system according to claim 15, wherein thetouch screen controller is arranged to transmit the synchronisationsignal to the proximity sensing system on the same connection as theproximity sensing signal.
 17. A system according to claim 1, furthercomprising a filtering capacitor arranged to be connected between one ofthe layers and ground during determination of the contact position, butdisconnected during measurement of said distance between a part of theuser and the touch screen.
 18. A touch screen control system comprisinga touch screen having first and second conductive layers arranged to bebrought together by touching of the screen, and a detection systemarranged to detect a contact position at which the screen is touched bymonitoring electrical signals from at least one of the layers, whereinthe first layer is arranged to act as a receiving antenna to receive aproximity sensing signal transmitted from a transmitting antenna via auser, and the detection system is further arranged to transmit thereceived proximity sensing signal on to a proximity sensing systemthereby to enable the proximity sensing system to determine a distancebetween a part of the user and the touch screen.
 19. (canceled)